St John’s wort and warfarin: why this herbal remedy is best avoided

St John’s wort is a herbal treatment that some people take for depression. The NHS notes that it is available from health food shops and pharmacies, and that there is some evidence it may help less severe depression. Because it is sold as a herbal product, many people assume it is harmless.

The NHS depression guidance is more cautious. It says doctors do not recommend it, because the amount of active ingredient varies between individual brands and batches, so you can never be sure what sort of effect it will have on you. It also says taking it with other medicines can cause serious problems.

Natural does not mean inert. Plant products contain active chemicals, and some of them change how the body handles prescribed medicines. That is exactly the situation with warfarin.

Warfarin is a narrow-window medicine. The NHS says regular INR blood tests are used to make sure you are on the right dose, and that your dose may change based on the results. The BNF interaction list for warfarin records that St John’s wort decreases the anticoagulant effect of warfarin. In practical terms, the same dose of warfarin does less, and the INR may fall below the target range.

A falling INR matters because warfarin is prescribed to prevent clots, for example in deep vein thrombosis, pulmonary embolism, atrial fibrillation or a mechanical heart valve, as the NHS lists. If protection weakens, the risk of a clot rises. The BNF rates this entry as moderate, based on theoretical evidence, but the NHS advice to avoid the herbal remedy is unambiguous.

There is a second hazard. People who take St John’s wort on and off, or who stop it after a period of use, can have a varying effect on their warfarin. Whenever a product that interacts is started or stopped, the INR can change, and that is hard to manage if the clinic does not know.

Do not simply stop in secret, and do not stop your warfarin. Tell the anticoagulation clinic or GP what you have been taking, how much and for how long. They can decide whether an extra INR test is needed and whether the warfarin dose needs reviewing as the herbal product is withdrawn.

Because the strength of herbal products varies, it helps to bring the packet. If you were taking it for low mood, that symptom still needs attention, which is covered below.

• Tell the clinic that manages your warfarin before you change anything.

• Bring the product packaging so the ingredients and dose can be seen.

• Ask whether an earlier INR test is sensible after you stop.

• Keep your anticoagulant alert card up to date with all products you use.

Warfarin is not the only concern. The NHS clopidogrel page says St John’s wort can increase the level of clopidogrel in the blood, which can increase the risk of bleeding. The NHS sertraline page says not to use it with sertraline, and the BNF notes that sertraline with St John’s wort might increase the risk of serotonin toxicity. The NHS depression page also says it can reduce the effectiveness of the contraceptive pill.

The pattern is that the herbal product interacts with several types of medicine in different ways, some weakening and some strengthening effects. Anyone on regular prescription medicines should assume a check is needed before adding it.

If low mood led you to St John’s wort, a GP can discuss options. The NHS describes talking therapies, antidepressant medicines, exercise, mindfulness and other approaches in its depression guidance. Which suits you depends on how severe your symptoms are and what else you take, and warfarin is a factor that the prescriber will take into account.

Some antidepressants also interact with warfarin, as the BNF lists for sertraline, citalopram, fluoxetine and paroxetine, which are noted as increasing bleeding risk when combined. That is not a reason to avoid treatment; it is a reason to make the choice with someone who knows your full list of medicines.

A weaker warfarin effect may produce no symptoms until a clot forms. A painful, swollen leg, sudden breathlessness, chest pain or symptoms of stroke need urgent help; call 999. If you have symptoms of bleeding or a head injury while on warfarin, the NHS says to call NHS 111.

In the NHS, warfarin is usually managed by an anticoagulation clinic or GP surgery. If you buy herbal products privately, from pharmacies or online, the same rule applies: tell the clinic before starting or stopping them.